Andre, M

Eriksson, AI

Abstract [en]

We present a comprehensive overview of the electron density and six different wave types in the range similar to1 Hz to similar to1 MHz, and we investigate their occurrence, average wave frequency and amplitude as a function of location, Kp index, and solar illumination. Twenty-one months of Freja observations from the Northern Hemisphere obtained at similar to1700 km altitude and invariant latitudes 40degrees-75degrees are used. We find that waves around the lower hybrid frequency occur in one low-latitude dayside band and one high-latitude nightside band. The latter band correlates with precipitating auroral electrons and coexists with electromagnetic ion cyclotron (EMIC) waves. This indicates the importance of energetic electrons for the wave generation. Both broadband ELF waves and broadband high-frequency whistler mode waves are found at high latitudes, but whistler mode emissions are most common in regions of high electron densities on the dayside, while broadband ELF waves are found where the density is reduced on the nightside. Moreover, the average density in the presence of broadband ELF waves is more reduced when the ionosphere is dark than when it is sunlit. However, broadband whistler mode waves, Langmuir waves, and waves with an upper cutoff just below the proton gyrofrequency coincide with density enhancements when the ionosphere is dark. Ion heating correlated with auroral electrons coexists with EMIC waves and the high-latitude band of waves around the lower hybrid frequency. Furthermore, ion heating not correlated with downgoing electrons coexists with broadband ELF waves.